Condensation polymers exhibiting nonlinear optical response

ABSTRACT

This invention provides novel isotropic condensation polymers which exhibit nonlinear optical response, and which have utility as a transparent optical component in optical light switch an light modulator devices. 
     An invention isotropic polyester is illustrated by the following structure: ##STR1##

CROSS-REFERENCES TO RELATED APPLICATIONS

The subject matter of this patent application is related to that disclosed in patent application Ser. No. 898,982, filed Aug. 22, 1986; patent application Ser. No. 106,301, filed Oct. 9, 1987; patent application Ser. No. 120,253, filed Nov. 10, 1987; and patent application Ser. No. 148,262, filed Jan. 25, 1988.

BACKGROUND OF THE INVENTION

It is known that organic and polymeric materials with large delocalized π-electron systems can exhibit nonlinear optical response, which in many cases is a much larger response than by inorganic substrates.

In addition, the properties of organic and polymeric materials can be varied to optimize other desirable properties, such as mechanical and thermoxidative stability and high laser damage threshold, with preservation of the electronic interactions responsible for nonlinear optical effects.

Thin films of organic or polymeric materials with large second order nonlinearities in combination with silicon-based electronic circuitry have potential as systems for laser modulation and deflection, information control in optical circuitry, and the like.

Other novel processes occurring through third order nonlinearity such as degenerate four-wave mixing, whereby real-time processing of optical fields occurs, have potential utility in such diverse fields as optical communications and integrated circuit fabrication.

Of particular importance for conjugated organic systems is the fact that the origin of the nonlinear effects is the polarization of the π-electron cloud as opposed to displacement or rearrangement of nuclear coordinates found in inorganic materials.

Nonlinear optical properties of organic and polymeric materials was the subject of a symposium sponsored by the ACS division of Polymer Chemistry at the 18th meeting of the American Chemical Society, September 1982. Papers presented at the meeting are published in ACS Symposium Series 233, American Chemical Society, Washington, D.C. 1983.

The above recited publications are incorporated herein by reference.

Of more specific interest with respect to the present invention embodiments is prior art relating to polymers with comb-like side chains. Eur. Polym. J., 18, 651(1982) describes liquid crystalline polymers of the smectic and nematic types with cyanobiphenyl groups in the side chain: ##STR2## where R is hydrogen or methyl, n is an integer of 2-11, and X is an oxy, alkylene or carbonyloxy divalent radical.

SPIE Vol. 682, pages 56-64, Molecular and Polymeric Optoelectronic Materials: Fundamentals and Applications (presented at Aug. 21-22, 1986 meeting) describes liquid crystalline polymeric systems which are copolymers of a mesogenic monomer and a nonlinear optically responsive monomer.

A disadvantage of liquid crystalline polymers which exhibit mesogenic side chain nonlinear optical response is an observed light scattering effect when the polymer is in the form of a solid phase optical medium, e.g., the polymer medium exhibits more than about 20 percent scattering of transmitted incident light. The light scattering is due to deviations from ideal molecular order which accommodate defects that are not optically clear.

There is continuing interest in the theory and practice of polymers which are characterized by comb-like side chain structures which can be oriented in an applied external field.

There is also an increasing research effort to develop new nonlinear optical organic systems for prospective novel phenomena and devices adapted for prospective novel phenomena and devices adapted for laser frequency conversion, information control in optical circuitry, light valves and optical switches. The potential utility of organic materials with large second order and third order nonlinearities for very high frequency application contrasts with bandwidth limitations of conventional inorganic electrooptic materials.

Accordingly, it is an object of this invention to provide novel polymers with comb-like side chains.

It is another object of this invention to provide condensation polymers having side chains which exhibit nonlinear optical response.

It is a further object of this invention to provide optical light switch and light modulator devices with a transparent polymeric nonlinear optical component comprising an isotropic polyester.

Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.

DESCRIPTION OF THE INVENTION

One or more objects of the present invention are accomplished by the provision of an isotropic condensation polymer which is characterized by recurring monomeric units corresponding to the formula: ##STR3## where m is an integer of at least 3; n is an integer between about 1-20; R is a covalent bond, or a divalent aliphatic, alicyclic or aromatic substituent containing between about 1-12 carbon atoms; and Z is a nitro or cyano substituent.

In another embodiment this invention provides a transparent nonlinear optical medium comprising an isotropic acrylic copolymer having a structure as represented in the above formula.

In another embodiment this invention provides a transparent nonlinear optical medium comprising an isotropic condensation polymer having a structure as represented in the above formula, and being further characterized by an external field-induced orientation of aligned pendant biphenyl side chains.

In another embodiment this invention provides an isotropic polyester which is characterized by recurring monomeric units corresponding to the formula: ##STR4## where m¹ is an interger between about 5-300; n¹ is an integer between about 2-6; and R¹ is a divalent C₁ -C₄ alkylene or phenylene substituent.

In another embodiment this invention provides an isotropic polyester which is characterized by recurring monomeric units corresponding to the formula: ##STR5## where m¹ is an integer between about 5-300; n¹ is an integer between about 2-6; and R¹ is a divalent C₁ -C₄ alkylene or phenylene substituent.

In a further embodiment this invention provides an optical light switch or light modulator device with a polymeric nonlinear optical component comprising a transparent solid medium of an isotropic polyester which is characterized by recurring monomeric units corresponding to the formula: ##STR6## where m is an integer of at least 3; n is an integer between about 1-20; R is a covalent bond, or a divalent aliphatic, alicyclic or aromatic substituent containing between about 1-12 carbon atoms; and Z is a nitro or cyano substituent.

The R substituent in the above defined formulae is illustrated by divalent aliphatic, alicyclic and aromatic radicals which include C₁ -C₁₀ alkylene, C₂ -C₁₀ alkylene, cyclopentylene, cyclohexylene, phenylene, 2-methoxyphenylene, 2-chlorophenylene, naphthylene, biphenylene, and the like.

A present invention isotropic polyester normally has a glass transition temperature in the range between about 40°-180° C., and a weight average molecular weight in the range between about 5000-200,000.

A present invention isotropic polyester has a glass-like appearance which is optically transparent in both solid and melt phases. An invention polymer is tractable, and the relatively low viscosity of the melt phase facilitates induced orientation of the polymer side chains by means of an external field.

The term "isotropic" as employed herein refers to a polyester which in the form of a transparent medium exhibits optical properties which are equivalent in all tensor directions.

The term "transparent" as employed herein refers to an optical medium which is transparent or light transmitting with respect to incident fundamental light frequencies and created light frequencies. In a nonlinear optical device, a present invention polymeric nonlinear optical component is transparent to both the incident and exit light frequencies, and the polymeric nonlinear optical component exhibits less than about 15 percent scattering of transmitted incident light.

A present invention optical light switch or light modulator device typically will have a polymeric nonlinear optical component which is a transparent solid medium of an isotropic polyester having a stable orientation of an external field-induced alignment of pendant biphenyl side chains.

Illustrative of a present invention optical device containing a polymeric nonlinear optical component as defined above is a laser frequency converter, an optical Pockels effect device, an optical Kerr effect device, a degenerate four wave mixing device, an optical interferometric waveguide gate, a wide-band electrooptical guided wave analog-to-digital converter, an all-optical multiplexer, an all-optical demultiplexer, an optical bistable device, or an optical parametric device.

Optical harmonoic generating devices are described in U.S. Pat. Nos. 3,234,475; 3,395,329; 3,694,055; 3,858,124; and 4,536,450.

Optical Kerr effect devices are described in U.S. Pat. Nos. 4,428,873 and 4,515,429; and references cited therein.

Degenerate four wave mixing optical devices are discussed by Y. R. Shen in Chapter 15, "The Principles Of Nonlinear Optics"; John Wiley & Sons, New York (1984). A nonresonant degenerate four wave mixing mirror device is described by J. Feinberg et al in Optics Letters, 5(12), 519(1980).

An optical interferometric waveguide gate device is described by A. Lattes et al in IEEE J. Quantum Electron, QE-19(11), 1718(1983).

A wide-band electrooptical guided-wave analog-to-digital converter device is described by R. A. Becker et al in Proceedings Of the IEEE, 72(7), 802(1984).

Optical multiplexer-demultiplexer devices are described in U.S. Pat. Nos. 3,532,890; 3,755,676; 4,427,895; 4,455,643; and 4,468,776.

Optical bistable devices are described in U.S. Pat. Nos. 4,515,429 and 4,583,818; and by P. W. Smith et al in Applied Physics Letters, 30(6), 280(1977), and in IEEE Spectrum, June 1981.

Optical parametric devices are described in U.S. Pat. Nos. 3,371,220; 3,530,301; and 3,537,020.

A present invention optical device can be provided by constructing an optical device of the type described in the technical literature, except that a novel isotropic polyester as defined herein is utilized as the nonlinear optical component.

Synthesis Of Isotropic Condensation Polymers

The preparation of isotropic polyesters with nonlinear optically responsive side chains is illustrated by the following flow diagram: ##STR7##

Nonlinear Optical Properties

The fundamental concepts of nonlinear optics and their relationship to chemical structures can be expressed in terms of dipolar approximation with respect to the polarization induced in an atom or molecule by an an external field.

As summarized in the ACS Symposium Series 233(1983) listed hereinabove in the Background Of The Invention section, the fundamental equation (1) below describes the change in dipole moment between the ground state μ_(g) and an excited state μ_(e) expressed as a power series of the electric field E which occurs upon interaction of such a field, as in the electric component of electromagnetic radiation, with a single molecule. The coefficient α is the familiar linear polarizability, β and γ are the quadratic and cubic hyperpolarizabilities, respectively. The coefficients for these hyperpolarizabilities are tensor quantities and therefore highly symmetry dependent. Odd order coefficients are nonvanishing for all structures on the molecular and unit cell level. The even order coefficients such as β are zero for those structures having a center of inversion symmetry on the molecular and/or having a center of inversion symmetry on the molecular and/or unit cell level.

Equation (2) is identical with (1) except that it describes a macroscopic polarization, such as that arising from an array of molecules in an isotropic polymer domain:

    Δμ=μ.sub.e -μ.sub.g =αE+βEE+γEEE+(1)

    P=P.sub.O +χ.sup.(1) E+χ.sup.(2) EE+χ.sup.(3) EEE+(2)

Light waves passing through an array of molecules can interact with them to produce new waves. This interaction may be interpreted as resulting from a modulation in refractive index or alternatively as a nonlinearity of the polarization. Such interaction occurs most efficiently when certain phase matching conditions are met, requiring identical propagation speeds of the fundamental wave and the harmonic wave. Birefringent crystals often possess propagation directions in which the refractive index for the fundamental ω and the second harmonic 2ω are identical so that dispersion may be overcome.

The term "phase matching" as employed herein refers to an effect in a nonlinear optical medium in which a harmonic wave is propagated with the same effective refractive index as the incident fundamental light wave. Efficient second harmonic generation requires a nonlinear optical medium to possess propagation directions in which optical medium birefringence cancels the dispersion as a function of wavelength, i.e., the optical transmission of fundamental and second harmonic frequencies is phase matched in the medium. The phase matching can provide a high conversion percentage of the incident light to the second harmonic wave.

For the general case of parametric wave mixing, the phase matching condition is expressed by the relationship:

    n.sub.1 ω.sub.1 +n.sub.2 ω.sub.2 =n.sub.3 ω.sub.3

where n₁ and n₂ are the indexes of refraction for the incident fundamental radiation, n₃ is the index of refraction for the created radiation, ω₁ and ω₂ are the frequencies of the incident fundamental radiation and ω₃ is the frequency of the created radiation. More particularly, for second harmonic generation, wherein ω₁ and ω₂ are the same frequency ω, and ω₃ is the created second harmonic frequency 2 ω, the phase matching condition is expressed by the relationship:

    n.sub.ω =n.sub.2ω

where n.sub.ω and n₂ω are indexes of refraction for the incident fundamental and created second harmonic light waves, respectively. More detailed theoretical aspects are described in "Quantum Electronics" by A. Yariv, chapters 16-17 (Wiley and Sons, New York, 1975).

A present invention isotropic condensation polymer medium typically has excellent optical transparency and exhibits hyperpolarization tensor properties such as second harmonic and third harmonic generation, and the linear electrooptic (Pockels) effect. For second harmonic generation, the bulk phase of the condensation polymer medium whether liquid or solid does not possess a real or orientational average inversion center. The substrate is a macroscopic noncentrosymmetric structure.

Harmonic generation measurements relative to quartz can be performed to establish the value of second order and third order nonlinear susceptibility of the optically clear substrates.

In the case of macroscopic nonlinear optical media that are composed of noncentrosymmetric sites on the molecular and domain level, the macroscopic second order nonlinear optical response χ.sup.(2) is comprised of the corresponding molecular nonlinear optical response β. In the rigid lattice gas approximation, the macroscopic susceptibility χ.sup.(2) is expressed by the following relationship:

    χ.sub.ijk (-ω.sub.3 ; ω.sub.1,ω.sub.2)=Nf.sup.ω.sbsp.3.sup.f.sup.ω.sbsp.2 f.sup.ω.sbsp.1 <β.sub.ijk (-ω.sub.3 ; ω.sub.1,ω.sub.2)<

wherein N is the number of sites per unit volume, f represent small local field correlations, β_(ijk) is averaged over the unit cell, ω₃ is the frequency of the created optical wave, and ω₁ and ω₂ are the frequencies of the incident fundamental optical waves.

A nonlinear optical medium with a centrosymmetric configuration of polymer molecules as defined herein can exhibit third order nonlinear optical susceptibility χ.sup.(3) of at least about 1×10⁻¹⁰ esu as measured at 1.91 μm excitation wavelength.

A nonlinear optical medium with an external field-induced noncentrosymmetric configuration of polymer molecules as defined herein can exhibit second order nonlinear optical susceptibility χ.sup.(2) of at least about 5×10⁻⁸ esu as measured at 1.91 μm excitation wavelength.

These theoretical considerations are elaborated by Garito et al in chapter 1 of the ACS Symposium Series 233 (1983); and by Lipscomb et al in J. Chem., Phys., 75, 1509 (1981), incorporated by reference. See also Lalama et al, Phys. Rev., A20, 1179 (1979); and Garito et al, Mol., Cryst. and Liq. Cryst., 106, 219 (1984); incorporated by reference.

External Field-Induced Side Chain Orientation

The term "external field" as employed herein refers to an electric, magnetic or mechanical stress field which is applied to a medium of mobile organic molecules, to induce dipolar alignment of the molecules parallel to the field.

The nonlinear optically responsive side chains of a present invention polyester may be aligned by the application of an external field to a mobile matrix of the acrylic copolymer molecules. Application of a DC electric field produces orientation by torque due to the interaction of the applied electric field and the net molecular dipole moment of the polymer side chains. The molecular dipole moment is due to both the permanent dipole moment (i.e., the separation of fixed positive and negative charge) and the induced dipole moment (i.e., the separation of positive and negative charge by the applied field).

Application of an AC electric field also can induce bulk alignment. In this case, orienting torque occurs solely due to the interaction of the applied AC field and the induced dipole moment. Typically, AC field strengths exceeding 1 kV/cm at a frequency exceeding 1 KHz are employed.

Application of a magnetic field also can effect alignment. Organic molecules do not possess a permanent magnetic dipole moment. In a manner analogous to AC electric field, a magnetic field can induce a net magnetic dipole moment. Torque results from the interaction of the induced dipole moment and the external magnetic field. Magnetic field strengths exceeding 10 Kgauss are sufficient to induce alignment of mobile biphenyl side chains of a polyester.

Mechanical stress induced molecular alignment is applicable to side chain polyesters. Specific mechanical stress methods include stretching a thin film, or coating a polyester surface with an aligning polymer such as nylon. Physical methods (e.g., stretching) rely upon the rigid and geometrically asymmetric character of the polyester molecules to induce bulk orientation. Chemical methods (e.g., coating the surface with an aligning polymer) rely upon strong intermolecular interactions to induce surface orientation.

Application of an AC electric, magnetic or mechanical external field produces colinear molecular alignment in which the molecular direction (either parallel or antiparallel to the orientation axis) is statistically random, and the resultant molecularly oriented medium exhibits third order nonlinear optical susceptibility χ.sup.(3). Application of a DC electric external field produces colinear molecular alignment in which the molecular direction is not random, and is characterized by a net parallel alignment of molecular dipoles. The resultant molecularly oriented medium exhibits second order nonlinear optical susceptibility χ.sup.(2).

The orientation of the isotropic polyester side chain is accomplished when the polymer molecules are in a mobile phase, e.g., the polyester is at a temperature near or above the polymer glass transition temperature. The aligned phase of the mobile molecules can be frozen by cooling the medium below the glass transition temperature while the aligned phase is still under the influence of the applied external field.

The following examples are further illustrative of the present invention. The components and specific ingredients are presented as being typical, and various modifications can be derived in view of the foregoing disclosure within the scope of the invention.

EXAMPLE I

This Example illustrates the preparation of an isotropic 50/50 condensation polymer of 4-[N,N-bis(2-hydroxyethyl)amino]-4'-nitrobiphenyl with terephthalic acid in accordance with the present invention. ##STR8##

A. 4-[N,N-Bis(2-hydroxyethyl)amino]-4'-nitrobiphenyl

A one liter three necked flask fitted with a mechanical stirrer, condenser and thermometer is charged with 21.7 g of 4-fluoro-4'-nitrobiphenyl, 31.0 g of diethanolamine, 13.8 g of potassium carbonate, 1.0 g of Aliquot 336, and 500 ml of dimethyl sulfoxide. The reaction mixture is heated at 95° C. for a period of three days, cooled, and poured into 4 liters of ice water. The crude solid product is filtered, washed with water, and dried.

B. Copolymer with terephthalic acid

4-[N,N-Bis(2-hydroxyethylamino]-4'-nitrobiphenyl (3.02 g) and dimethyl terephthalate (1.94 g) are admixed in a reactor, and dried and degassed for one hour. To the mixture is added 3 drops of a 9:1 mixture of diglyme and tetraisopropyl orthotitanate under argon. Under a slow stream of argon, the melted mixture is stirred at 170° C. for a period of 2 hours. At the end of this period, 3 more drops of catalyst is added and the pressure is reduced to 1 mm over a period of 2 hours while the temperature is raised to 200° C. Subsequently, the pressure is reduced further to <0.1 mm, and the reaction mixture is heated an additional 2 hours. The crude polymer product is cooled, and dissolved in chloroform. The polymer then is precipitated by pouring the product solution into methanol, and the precipitate is recovered and dried.

The condensation polymer has a glass transition temperature of 55° C., and a weight average molecular weight in the range of 50,000-80,000.

EXAMPLE II

This Example illustrates the preparation of isotropic condensation polymers in accordance with the present invention.

Following the procedures of Example I, 4-[N,N-bis(2-hydroxyethylamino]-4'-nitrobiphenyl (3.02 g) is condensed with dimethyl succinate (1.0 g) and dimethyl maleate (0.5 g).

The recovered condensation polymer has a glass transition temperature of 60° C., and a weight average molecular weight in the range of 50,000-80,000.

Utilizing the same general procedures, the following isotropic condensation polymers are prepared which have a weight average molecular weight in the range of 50,000-80,000: ##STR9##

EXAMPLE III

This Example illustrates a poling procedure for producing a transparent film of an isotropic condensation polymer which exhibits second order nonlinear optical response in accordance with the present invention.

A. Poling Cell Construction

A poling cell is constructed from electrically conductive glass plates, such as Corning Glass EC-2301. The glass plates are washed with sulfuric acid, isopropanol, 1-dodecanol, and isopropanol, with a distilled water rinse between each washing step.

The poling cell is a sandwich type cell in which the conductive glass surfaces are in facing proximity and are separated by a polyimide film of approximately 25 micrometer thickness. A thin layer of epoxy adhesive is applied on the surfaces of the polyimide film to hold the glass plates.

After the epoxy is completely cured, the cell is washed with isopropanol and rinsed with distilled water. After drying, the cell is stored in a dry box.

B. Filling The Poling Cell

The condensation polymer of Example I is placed in a vacuum oven and maintained in a melt phase at a temperature of about 100° C. for about 2 hours to eliminate entrained air bubbles from the polymer melt.

The polymer melt is introduced into the space between the glass plates by charging a drop of the polymer melt to one of the openings of the poling cell space and placing the cell assembly in a vacuum oven maintained at a temperature of approximately 120° C. The cell space fills gradually by capillary action. The space filling period is about 2 hours for a 0.5 cm long space.

C. Electric Field-Induced Orientation

Two lead wires are attached to each of the conductive glass surfaces using electrically conductive epoxy adhesive. The poling assembly is placed in a microscope hot stage (Mettler FP-82 with FP-80 Central Processor), and the sample is observed with a polarizing microscope (Leitz Ortholux Pol) for alignment.

The microscope is switched into a photodiode (Mettler Photometer No. 17517) to record the change of light intensity upon application of an electric field. The two lead wires are connected to an AC voltage amplifier (Electro-Optic Developments LA10A), which amplifies the voltage signal from a signal generator (Hewlett-Packard No. 3310B).

The poling cell first is heated to 100° C. to bring the polymer to the melt phase. The AC voltage source is set at 500 V, and the frequency is set at 2000 Hz. The power to the poling cell is turned on to apply an electric field across the copolymer sample. The field strength is calculated to be approximately 2×10⁵ V/cm. About three seconds after the electric field is applied, the photodiode signal drops close to the baseline, indicating that orientation development induced by the electric field is completed. At this point, the cell is cooled to room temperature, and the poling assembly is disconnected from the power source.

When the poling assembly is removed from the microscope hot stage, by visual observation the polymer in the cell space is transparent. This is an indication that the molecular orientation is uniform and homogeneous throughout the sample. The polymer molecules possess no net alignment of molecular dipoles along the electric field even though the molecular axis is oriented along the field axis. Antiparallel packing of molecular dipoles produces a center of inversion symmetry so that the materials exhibit third order nonlinear optical susceptibility χ.sup.(3).

D. High Field Poling For Symmetry Control

The oriented isotropic condensation polymer sample is subjected to a higher electric field to develop a noncentrosymmetric orientation of nonlinear optical moieties which are a part of the side chains of the polymer.

The poling cell assembly is heated to 50° C., which is approximately 5° C. below the glass transition temperature of the copolymer. Then the lead wires of the poling assembly are connected to a DC voltage source (Kepco OPS-3500) and the voltage is turned up until it reaches 2000 V. At this point, the electric field strength across the sample is about 8×10⁵ V/cm. The sample is maintained at this field strength level for a period of about two seconds or longer as necessary to achieve the molecular alignment, and then the voltage source is disconnected. A noncentrosymmetrically oriented polymer matrix is obtained when the cell sample is cooled.

The χ.sup.(2) value for the polymer nominally is 10×10⁻⁹ esu as measured at 1.91 μm excitation wavelength. A comparative χ.sup.(2) value for potassium dihydrogen phosphate is 2.4×10⁻⁹ esu. 

What is claimed is:
 1. An isotropic condensation polymer which is characterized by recurring monomeric units corresponding to the formula: ##STR10## where m is an integer of at least 3; n is an integer between about 1-20; R is a covalent bond, or a divalent aliphatic, alicyclic or aromatic substituent containing between about 1-12 carbon atoms; and Z is a nitro or cyano substituent.
 2. A condensation polymer in accordance with claim 1 which has a weight average molecular weight in the range between about 5000-200,000.
 3. A condensation polymer in accordance with claim 1 which has a glass transition temperature in the range between about 40°-180° C.
 4. A transparent nonlinear optical medium comprising a condensation polymer in accordance with claim
 1. 5. A transparent nonlinear optical medium in accordance with claim 4 which is characterized by an external field-induced orientation of aligned pendant biphenyl side chains.
 6. An isotropic polyester which is characterized by recurring monomeric units corresponding to the formula: ##STR11## where m¹ is an integer between about 5-300; n¹ is an integer between about 2-6; and R¹ is a divalent C₁ -C₄ alkylene or phenylene substituent.
 7. A polyester in accordance with claim 6 wherein n¹ is 2, and R¹ is para-phenylene.
 8. A transparent nonlinear optical medium comprising a polyester in accordance with claim
 6. 9. A transparent nonlinear optical medium in accordance with claim 8 which is characterized by an external field-induced orientation of aligned pendant biphenyl side chains.
 10. An isotropic polyester which is characterized by recurring monomeric units corresponding to the formula: ##STR12## where m¹ is an integer between about 5-300; n¹ is an integer between about 2-6; and R¹ is a divalent C₁ -C₄ alkylene or phenylene substituent.
 11. A polyester in accordance with claim 10 wherein n¹ is 2, and R¹ is para-phenylene.
 12. A transparent nonlinear optical medium comprising a polyester in accordance with claim
 10. 13. A transparent nonlinear optical medium in accordance with claim 12 which is characterized by an external field-induced orientation of aligned pendant biphenyl side chains. 